1. Field of the Invention
The present invention relates to a sound multiplex signal encoder circuit, and more specifically to a sound multiplex signal encoder circuit in which a sub-channel signal is generated by modulating a sub-channel source sound signal.
2. Description of the Related Art
The sound multiplex modulation system of a television processes, for example, a sound multiplex signal in which a main channel signal, a sub-channel signal, and a multiplex control signal are superimposed. The sub-channel signal is obtained by frequency-modulating a sub-carrier wave with a sub-channel source sound signal. The frequency of the sub-carrier wave is set 2 fh higher than that of the main carrier wave, where fh is the frequency of a horizontal synchronizing signal, thereby to minimize the amount of buzz interruption due to the presence of a video signal, which is caused by the sound intercarrier system of a receiver.
Generally, in a sound multiplex signal encoder circuit used for sound multiplex broadcast, a PLL circuit controls the frequency of the sub-carrier wave, using a horizontal synchronizing signal included in a video signal as a reference signal. Hence, when no video signals are input, or when no horizontal synchronizing signals are detected for some reason, the frequency of the sub-carrier wave changes, inevitably causing the intercarrier signal and the 2 fh component of the video signal to generate beats in the receiver. Consequently, the receiver produces sweep sound while outputting sound.